I hate to be the bearer of bad news but, typically you do not have any legal recourse when you purchase a used car out of warranty and it has mechanical issues. It falls under caveat emptor, or in non legal jargon, it is the buyer's problem if anything goes wrong. The only real exception to this would be if they lied to you and said something along the lines of it had a new clutch. Some states have extensions of the core federal lemon laws that cover used cars in some way. Unfortunately I do not think NC covers used cars under their lemon law.
Be careful about making threats against them if you do not have legal grounds. They would be within legal rights to bring a defamation case against you for slander and/or libel.
Autopedia has a good over view of sites that cover lemon laws:
http://autopedia.com/html/HotLinks_Lemon.html